The present invention relates generally to fuel systems and more particularly relates to fuel metering systems for aircraft engines.
Most engines utilize a fuel supply and a fuel system for regulating the flow of fuel from the fuel supply to the combustion chamber of the engine. In a typical gas turbine engine. the fuel system comprises a fuel pump for pressurizing fuel from the fuel supply and a fuel metering system for controlling the flow of fuel to the combustion chamber. Typically the fuel metering system comprises a metering valve for modulating the fuel flow rate to the engine, while a pressurizing or shutoff valve ultimately denies or permits the delivers fuel to the combustion chamber. The system may further include a by pass valve for regulating the pressure drop across the metering valve. An important characteristic of the fuel system is the ability to accurately control the volumetric rate of fuel flow to the combustion chamber.
Some fuel metering systems control the fuel flow rate by utilizing electronic feedback to control the position of the metering valve. In one such system. a portion of the fuel from the pump flows through a flapper-type servo valve which produces a controllable output pressure which in turn controls the position of the metering valve. The metering valve responds by controlling its valve opening which directly meters fuel flow to the engine. The servo valve is operated as a function of the applied electrical current, and its output, which has a level which is precisely related to the input current, regulates the much larger flow of fuel through the metering valve. The fuel flow through the servo valve is returned to a point in the system upstream of the fuel pump for recirculation and thus is considered xe2x80x9cparasitic leakagexe2x80x9d. An electronic position sensing device is coupled to the metering valve and determines the physical position of the metering valve. the position giving the flow rate of the fuel flowing through the metering valve. The sensing device sends an electrical signal to an electronic engine controller, which in turn generates a signal for the servo valve to regulate the throttling flow.
Systems of this type are characterized by requiring a small but potentially significant flow of fuel through the servo valve which is used only for control. The current applied to the servo valve regulates the flow through the valve which in turn regulates the output of the valve (typically in the form of a pressure) in turn directly controls the position of the metering valve. At engine start, where fuel flow to the engine is at relatively low rates. the control flow can become a significant part of the total. At all fuel flow rates, the control flow is a parasitic loss in that it is not directly involved in causing the engine to supply power. In these systems, the metering valve alone regulates the fuel flow ultimately supplied to the engine, and can therefore be termed a single-stage fuel metering system. From start to maximum engine speed, the metering valve controls the rate of fuel flow delivered to the combustion chamber, by virtue of a position sensor, electronic controller and servo valve.
While such systems have enjoyed much success, there are some drawbacks to these single-stage fuel metering systems. The range of fuel flow to be regulated by these systems ranges from about 5 pph (pounds per hour) at idle to no upper limit at maximum engine speed since the upper limit of the range depends on the particular engine used. The amount of fuel flow required for engine start ranges from about 20 pph to 250 pph, which is a relatively small range relative to the entire flow rate range which can be 5 pph to 5000 pph and higher, for example. It can therefore be seen that a single-stage metering valve that is sized to regulate a large range of fuel flows becomes less precise when regulating a smaller range of flow rates, such as during start-up.
Further, the portion of the fuel pressurized by the pump that is used to throttle the metering valve is not delivered to the engine. Rather, the control fuel flow is returned upstream for re-circulation, and causes a parasitic leakage within the fuel metering system. This is because the pump must not only pressurize the requisite fuel to be supplied to the combustion chamber of the engine, but must also pressurize the requisite fuel to control the metering valve, resulting in the need for a larger high-pressure pump. A larger pump will generate more heat which must be dissipated by the fuel system, an obvious concern in aircraft engines. A larger heat exchanger may be required to dissipate the heat. Moreover, size, weight and cost are of obvious importance in engine and fuel system design, and a larger pump increases the size and weight of the fuel system. Similarly the hardware and controls needed for an electronic feedback system increase the size, weight and cost of the fuel system.
In light of the above, a general object of the present invention is to provide a simplified, yet highly accurate, two-stage fuel metering system for an engine.
In that regard, it is also an object of the present invention to provide a fuel metering system that is highly accurate and precise at low flow rates, particularly at engine start awhile still capable of supplying accurately metered high flow rates when demanded.
It is a similar object of the present invention to provide a fuel metering system that regulates fuel in two different modes, one for low flow rates and another for high flow rates.
It is a related object of the present invention to provide a fuel metering system that does not require electronic feedback for accurate fuel flow regulation.
It is another object of the present invention to minimize the parasitic leakage in the fuel metering, system.
It is a further object of the present invention to provide a fuel metering system that is low cost, light weight and of minimum size.
In accordance with these objects, the present invention provides a novel two-stage fuel metering system for metering the flow of fuel between a fuel supply and a combustion chamber of an engine. Generally, two distinct flow paths are provided for pressurized fuel flowing to the engine. A servo valve regulates the flow through the first flow path and a metering valve regulates the flow through the second flow path, the metering valve being hydraulically controlled by the servo valve.
It is therefore a feature of the present invention that the two distinct flow paths result in two different modes of regulating the fuel supplied to the engine. At low flow rates, the servo valve regulates the flow of fuel supplied to the engine. At higher flow rates, the servo valve hydraulically controls the metering valve to regulate the flow of fuel supplied to the engine.
According to certain more specific respects, it is also a feature of the present invention to provide a first primary passageway for receiving fuel from a fuel supply and a second primary passageway for delivering fuel to an engine. The two flow paths connect the first primary passageway to the second primary passageway. The first flow path includes a servo chamber fluidically connected to both the first and second primary passageways, and the servo valve selectively regulates the flow of fuel entering the chamber from the first passageway. The metering valve forms the second flow path and fluidically connects the first and second passageways to regulate the flow of fuel from the first passageway to the second passageway. The metering valve is biased to a closed position that obstructs the fuel flow, and is disposed in the servo chamber such that it responsive to the pressure in the chamber. The metering valve is adapted to move from the closed position toward an open position in response to sufficient fuel pressure in the chamber. The first and second flow paths are thus coupled in parallel between the first and second passageways to form the novel two-stage fuel metering system.
The flow rate of fuel through the first flow path (regulated by the servo valve) is limited to a predetermined level. At fuel flow rates below the predetermined level, i.e. low flow rates, pressurized fuel follows the first flows path and travels from the first passageway through the servo valve and chamber into the second passageway for delivery to the engine. As the servo valve allows fuel to enter the chamber at a rate higher than can pass through the first flow path, the pressure in the chamber builds. When the pressure reaches an adequate level, the actuating surface of the metering valve responds to the pressure and the metering valve moves toward an open position, allowing fuel to begin flowing directly from the first passageway to the second passageway, through the metering valve. The fuel metering system is therefore two-staged, one flow of fuel flows through the servo valve to the second passageway, at a rate determined by the servo valve, while a second flows of fuel flows through the metering valve to the second passageway at a rate determined by the position of the metering valve. As the servo valve increases the rate of fuel flowing into the chamber, the pressure in the chamber increases and the metering valve opens further, to pass more fuel through to the second passageway and hence the combustion chamber of the engine. Therefore, the current applied to the servo valve determines both the flow through the first flow path, as well as the flow through the second flow path.
It is another feature of the present invention to provide mechanical feedback between the metering valve and the servo valve. The servo valve regulates the flow of fuel therethrough as a function of electrical current applied to the servo valve. The mechanical feedback reduces the increase in fuel flow into the chamber that results from increasing the current applied to the servo valve once the metering valve has been opened. The mechanical feedback therefore increases the precision of control over positioning the metering valve, and hence the fuel flow rate to the combustion chamber.
These and other object and advantages of the invention will become more apparent from the following detailed description when taken in con junction with the accompanying drawings.